FXUS63 KLSX 061140
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
540 AM CST SUN DEC 6 2009

.DISCUSSION...
/525 AM CST SUN DEC 6 2009/

THE LONGWAVE H500 TROF THAT HAS BEEN ADVERTISED TO RETROGRADE HAD 
THIS PROCESS WELL UNDERWAY FOR THE 00Z RAOBS LAST EVENING AND BASED 
ON THE LATEST WATER VAPOR IMAGERY...IT HAS BEEN ACHIEVED. THIS IS 
IMPORTANT BECAUSE THE DEGREE OF DEEPENING WILL PLAY A KEY FACTOR IN 
THE MAJOR WINTER STORM THAT IS EXPECTED TO TAKE SHAPE ACROSS THE 
MIDWEST TUESDAY AND WEDNESDAY. THE MODELS SHOULD BEGIN TO GET A LOCK 
ON THE TRACK LATER TODAY OR MORE LIKELY...BY MONDAY. IN THE  
MEANTIME...WE ARE STILL EXPECTING A QUICK MOVING SHORTWAVE TO IMPACT 
OUR AREA TONIGHT THRU A MEAN ZONAL FLOW PATTERN. A STRONGER...AND 
WELL ADVERTISED...SHORTWAVE WILL THEN DRIVE THRU THE MIDWEST ON 
TUESDAY AND INTO WEDNESDAY...AND WILL BRING WITH IT...AN EWD 
PROGRESSION OF THE LONGWAVE H500 TROF AS IT RE-ESTABLISHES ITSELF 
OVER THE CNTRL CONUS. THIS LONGWAVE TROF WILL THEN RETREAT SOMEWHAT 
HEADING INTO THE END OF THE WORK WEEK. 

HAVE SIDED WITH THE COOLER METMOS TEMPS ONCE AGAIN FOR TODAY/S MAXES 
AS TEMPS ALOFT ARE NOT MUCH DIFFERENT THAN PERSISTENCE...BUT THE 
FACTOR THAT HAS CHANGED IS THICKENING CLOUDS OVERHEAD AND WILL 
PROBABLY RESULT IN TEMPS NEAR PERSISTENCE.

THE SYSTEM FOR TONIGHT WILL BE QUICK MOVING AND PRESENT LIFT IN TWO 
STAGES. THE FIRST WILL BE ASSOCIATED WITH THE UPPER LEVEL SYSTEM AND 
PRIMARILY ACT ON THE AIR OVER THE NRN FA...WHILE THE SECOND WILL BE 
ASSOCIATED WITH WAA AT THE LOWER LEVELS AND WILL ALSO CARRY WITH IT 
A TONGUE OF BETTER MOISTURE. NE'ER TWAIN THE TWO SHALL MEET WHICH 
WILL LEAVE THEM WITH THEIR OWN OBSTACLES TO OVERCOME...THE DRY AIR 
AT THE LO LEVELS IN THE NRN FA...AND THE LACK OF DEEPER LIFT IN THE 
SERN FA. BELIEVE THE SERN FA WILL HAVE THE BEST CHANCE FOR 
MEASURABLE PCPN DUE TO LIFT CENTERED LOWER IN THE ATMOSPHERE BUT 
BRIEF PERIOD WHEN EVERYTHING COMES TOGETHER WILL RESULT IN A LOW 
QPF. MODEL SOUNDINGS HAVE COME IN A BIT COOLER THAN 24HRS AGO AND 
FAVOR MORE OF A SNOW P-TYPE WHEN THE ATMOSPHERE DOES SATURATE WHICH 
WILL LIKELY NOT HAPPEN IN MANY AREAS UNTIL CLOSE TO MIDNIGHT. BY 
LATE TONIGHT AND CONTINUING INTO MONDAY MORNING...WE ARE STILL 
EXPECTING MORE FAVORABLE CONDS FOR DZ/FZDZ DEPENDING ON THE PRECISE 
SFC TEMPS. 

MODELS CONTINUE TO SHOW A PREFERRED CONSENSUS FOR A NRN TRACK TO THE 
TUESDAY/WEDNESDAY STORM. AS A MATTER OF FACT...THE H850 LO TRACK 
WILL BARELY PUSH THRU THE FAR NWRN FA TUESDAY NIGHT...AND THIS 
STRONGLY SUGGESTS LITTLE IN THE WAY OF ACCUMULATING SNOW EXCEPT NEAR 
THIS TRACK AND EVEN THEN IT IS ON SHAKY GROUND BECAUSE OF TEMP 
PROFILE ISSUES...MEANING IT MAY NOT BE ENTIRELY ALL SNOW EVEN THERE. 
THE HEAVY SNOW TRACK APPEARS IT WILL BE STILL FURTHER NW INTO IOWA. 
HOWEVER...THE RETROGRADING LONGWAVE H500 TROF HAS NOT COMPLETED YET 
AND SO THIS TRACK COULD CERTAINLY CHANGE AND MEAN BIG CHANGES. 
ANOTHER ITEM IS THE STRENGTH OF THE BUILDING COLD AIRMASS FOR MONDAY 
AND HOW EASILY IT WILL GIVE WAY TO THIS STORM...WHICH MAY NUDGE IT 
FURTHER S. AS IT STANDS NOW...THOUGH...WE ARE LOOKING AT PRIMARILY 
SNOW IN THE FAR NWRN CORNER OF THE FA...NW OF KUIN...WHERE SEVERAL 
INCHES OF SNOW COULD ACCUMULATE. A PERIOD OF SNOW EARLY CHANGING TO 
RAIN FOR MUCH OF THE STORM THEN BACK TO SNOW LATE WILL BE IN ORDER 
FOR AREAS FURTHER S TO THE I-70 CORRIDOR. HOWEVER SFC TEMPS WILL 
LIKELY BE ABOVE FRZG FOR MUCH OF THE STORM IN AREAS THAT ARE S OF 
KUIN AND LIMIT ANY SNOW ACCUMS. WHAT MAKES THE P-TYPES CHALLENGING 
WILL BE THE WARM AIR SURGING OUT AHEAD OF THE STORM...EVEN THROUGH 
MUCH OF TUESDAY NIGHT...LIKELY RESULTING IN SLOWLY RISING TEMPS THRU 
TUESDAY EVENING WITH THE WEDGE OF WARM AIR ALOFT THAT WILL ALSO 
DEVELOP. WITH THE MORE NLY TRACK THE STORM APPEARS TO BE TAKING... 
CANNOT EVEN REALLY RULE OUT THUNDER FOR MANY AREAS TUESDAY NIGHT. 

ONCE THE STORM EXITS BY MIDDAY WEDNESDAY...MUCH COLDER AND WINDY 
CONDS WILL MOVE IN. LOOK FOR TEMPS TO FALL THRU THE DAY WITH MAX 
TEMPS AROUND 12Z/WED WITH GUSTS AS HI AS 40MPH. BLSN WILL BE AN 
ISSUE FOR AREAS THAT DO ACCUMULATE SEVERAL INCHES OF SNOW UP N. THE 
AIR WILL EVEN BE COLD ENOUGH BY LATE WED NIGHT AND THURSDAY MORNING 
TO BE TALKING WIND CHILLS APPROACHING 10 BELOW.

TES

&&

.AVIATION...
/539 AM CST SUN DEC 6 2009/

FOR THE 12Z TAFS...TAFS ARE SPLIT INTO TWO PARTS FOR THIS CYCLE. FIRST
HALF OF TAF IS A WIND FCST FOR TODAY AS SLY WINDS BACK TO BCMG ESELY
THIS AFTERNOON INTO THE EVE HOURS AS A SURFACE LOW APPROACHES THE REGION.
WINDS WILL THEN BCMG SLY AGAIN LATE TONIGHT...THEN SWLY TOWARD THE END
OF THE FORECAST PERIOD AS THE WEAK LOW DRIFTS NEWD THRU THE REGION.

THE SECOND HALF OF THE FORECAST DEALS WITH CIGS AND THE POTENTIAL OF
SOME LIGHT PRECIP LATE TONIGHT...MAINLY AT COU AND UIN. HAVE NOT MENTIONED
ANY PRECIP AT ANY SITE ATTM AS VISBYS ARE NOT EXPECTED TO BE REDUCED.
ANY PRECIP THAT DOES OCCUR IS EXPECTED TO START AS SN. AS DRY AIR IS
ADVECTED IN ABOVE THE CLOUD LAYER WITH THE TOP OF THE CLOUD LOWERING
BELOW THE DENDRITIC ZONE...PRECIP SHUD SWITCH OVER TO FZDZ. ANY PRECIP
WILL BE SHORT AS THE SYSTEM WILL BE MOVING QUICKLY THRU THE REGION.

TILLY

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
IL...NONE.
&&

$$

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